Disclosure, I have no specialized knowledge or experience. I wonder if he's already worked a deal with the investor? If he's offering the investor half of you

Disclosure, I have no specialized knowledge or experience. I wonder if he's already worked a deal with the investor? If he's offering the investor half of your stake to push you out (for example), you might be able to turn the tables and offer the investor 3/4 of his stake (or all of it) to push him out. This depends on your ability to have a frank conversation with the investor and their faith that you can continue to drive the company. If that's the case it also means the investor is probably toxic which doesn't bode well for the future relationship but it might at least prevent your co-founder from getting away with this.